Short spasms (usually less than a minute) of spewing lies undermining one's personal achievements and making those around the person feeling even more feeble with their pathetic accomplishments.
The original discovers assumed that these spasms were done on purpose, deliberate attacks on the esteem of others.
However, extensive observation of those with chronic Jenning Disease has proved that it is a serious medical condition

1)Person 1: look at all these african kids
Person 2:ugh.. i wish i were as skinny as them. im such a fatass sometimes
Person 1: stop jenning

2)When you have a 99% average and president of three clubs and councils at your school and you have volunteered at a third world country but worried about not getting hired at McDonalds.

To completely lose your mind over absolutely nothing in ten seconds or less. Process in which you blame all problems you have or ever will have on the closest available person or the person you think of first.

woman: "You're a cheating bastard!"

man "It's mother's day and I was with my mom!"
woman: "You've ruined my life and turned all of my friends against me!"
man: "Quit Jenning on me."